Answer Description

STP (Shielded Twisted Pair) is a type of Ethernet that has an EMI resistant jacket. Because there are a large amount servers, STP is necessary. Fiber is incorrect because the question specifies we need a cost effective solutions. Typically, fiber will be used on trunk and backbone connections.

Wikipedia

Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted balanced pair, a twisted pair reduces electromagnetic radiation, crosstalk between neighboring pairs and improves rejection of external electromagnetic interference. It was invented by Alexander Graham …